What effect does the downward movement of the piston have in a 4 stroke engine?

The downward movement of the piston in a 4-stroke engine is crucial during the intake stroke. As the piston descends in the cylinder, it creates a vacuum or low-pressure area that allows fresh air (and fuel, if it's a carbureted engine) to be drawn into the combustion chamber through the open intake valves. This is essential for the engine’s operation because it ensures that there is an adequate supply of the air-fuel mixture needed for combustion in the subsequent stroke.

This process highlights the importance of the piston’s movement in the overall cycle of an internal combustion engine. During the intake stroke, the vacuum is vital for initiating the replenishment of the chamber with air or a suitable fuel-air mixture, thus setting up the subsequent stages of compression and combustion. Other movements of the piston, such as during the power stroke, are focused on compressing the mixture or expelling exhaust gases, but it is the downward motion during the intake stroke that primarily contributes to the creation of a vacuum for drawing in fresh air.